The present invention relates generally to the field of medical devices and specifically to medical devices that are designed to monitor the respiratory characteristics of patient breathing, especially for those patients who are attached to mechanical ventilators.
Persons exhibiting acute or chronic respiratory failure, for example due to pulmonary infection or trauma, often require artificial ventilatory support and may therefore be connected, by means of flexible plastic ventilation tubing, to a mechanical ventilator. Correct functioning of such a ventilator entails continuous, accurate and reliable monitoring, by the ventilator, of airflow characteristics within the connecting plastic tubing. Such monitoring is often achieved by means of a gas-flow sensor interposed within the plastic ventilation tubing connecting the patient to the ventilator.
It will be well known to those familiar with the art that many mechanical ventilators utilize a flow sensor fashioned in the form of a bore of cylindrical tubing containing within it a strut, also known as an interfering body, in a manner which facilitates differential pressure measurements, at either end of the strut, that are proportional to the flow rate of the respiratory gases that pass through the sensor. Such a flow sensor is hereinafter referred to as a differential-pressure flow sensor, and is described in more detail below.
A differential-pressure flow sensor typically is comprised of a hollow cylindrical body having a bore, which can be connected between a ventilator and a patient. The differential-pressure flow sensor utilizes an aerodynamic strut that is disposed within the cylindrical bore of the sensor to create a drop in the pressure of the respiratory gases flowing through the sensor. The strut extends across the entire diameter of the bore of the flow sensor and bisects the circular bore of the sensor. The width of the strut is less than the diameter of the bore and the longitudinal length of the strut is less than the length of the bore. Further, the geometric cross section of the strut is symmetrical to the flow of respiratory gases flowing through the sensor in either direction, and has a generally elliptical cross section. The aerodynamic design of the strut preserves the laminar nature of airflow through the cylindrical bore as the air passes around the strut, such that airflow turbulence is minimal or absent within the flow sensor.
The aerodynamic strut has longitudinally exposed edge portions, such that when the differential-pressure flow sensor is interposed between a patient and a ventilator one edge portion is closer to the patient and the other edge portion is closer to the ventilator. Each of the edge portions of the aerodynamic strut contains a semicircular groove running the full height of the edge, that is, parallel to the short axis of the cylindrical body and extending from the inner surface of the cylindrical body on one side to the inner surface of the cylindrical body on the opposite side of the cylindrical body. One end of each groove is in continuity with a circular lumen within the wall of the cylindrical body. This circular lumen is thus located at the intersection between the inner surface of the cylindrical body and the edge portion of the aerodynamic strut. The circular lumen extends through the wall of the cylindrical body. On the outer surface of the cylindrical body this lumen receives tubing, which runs from the differential-pressure flow sensor to a pressure transducer, typically located within the mechanical ventilator.
The differential pressure measured by the flow sensor is due to the restriction to flow caused by the presence of the strut within the bore of the sensor. The drop in pressure is measured relatively between the groove in the first edge portion of the aerodynamic strut and the groove in the second edge portion of the aerodynamic strut. For example, when respiratory gases are flowing through the flow sensor from the first end to the second end, a high-pressure zone, also referred to as an area of static pressure, is created immediately adjacent to the first edge of the strut and a low-pressure zone, also referred to as an area of dynamic pressure, is created immediately adjacent to the second edge of the strut. The converse is true when the respiratory gases are flowing from the second end of the sensor toward the first end of the sensor. It should be emphasized that in terms of the functioning of the differential-pressure flow sensor, the actual point at which pressure is measured, using the venturi principle, is at the circular lumen on the inner surface of the cylindrical body, and that this pressure measurement reflects the pressure along the length of the groove in the edge portion of the aerodynamic strut.
The relative pressures of the respiratory gases flowing through the sensor are collected and conveyed to the pressure transducer through the circular lumens and the tubing connected thereto. The pressure transducer measures the received pressures, and the resultant data is then processed by a microprocessor so as to calculate the rate of gas flow through the differential-pressure flow sensor. This calculation is based on the principle that the drop in pressure across an obstruction in an airway is related to the square of the velocity of the fluids flowing through the airway. This principle is also true for the differential-pressure flow sensor. The general relationship between the flow velocity and the pressure drop as measured across the strut by the transducer is given by:(flow velocity)2 ΔP 
where ΔP is the drop in pressure across the strut of the differential-pressure flow sensor. This relationship is unique for every unique flow sensor geometry and must be derived empirically. Accordingly, the plastic flow sensors that are used with a given ventilator and microprocessor are manufactured from the same molds and injection conditions so that the geometric variation between each flow sensor is negligible.
Determining the flow-to-pressure drop relationship is accomplished by forcing air through the differential-pressure flow sensor at predetermined flow rates and measuring the resulting changes in differential pressure across the strut through the lumens, so as to generate a set of data points. A high order linear equation is then fit to the data points. This equation closely follows the same general form as given above. Using this equation, a flow velocity for gases flowing through the differential-pressure flow sensor can be calculated from the differential pressures measured across the strut.
It is known, however, that standard differential-pressure flow sensors, as described above, suffer from several deficiencies. In particular, standard differential pressure flow sensors have been found to function unreliably in the presence of high humidity within the respiratory gasses flowing through the sensor. Humidification of the inspired respiratory gasses is often achieved by flowing the respiratory gasses through a water humidifier before the gasses pass through the flow sensor and into the patient. This is desirable so as to prevent drying of the patient's respiratory tract mucousa during prolonged periods of mechanical ventilation. Humidity may also be introduced into the respiratory gasses in the form of aerosolized medications, which are frequently administered to mechanically ventilated patients. Even without the introduction of external humidity, the naturally expired respiratory gasses from a patient's lungs are of higher humidity than the inspired gasses, thus increasing the humidity of airflow through the differential-pressure flow sensor.
As the humidity of the respiratory gasses increases, water condensation may occur on the inner surface of the respiratory tubing and the differential-pressure flow sensor. When such condensation occurs in the circular lumen in the inner wall of the flow sensor, at which site pressure measurements are sensed, water blocks the lumen, thus distorting the pressure measurements recorded by the differential-pressure flow sensor, and invalidating the resultant flow data. Furthermore, water that has previously condensed elsewhere along the length of the flexible respiratory tubing may flow into the flow sensor due to movement of the tubing, and cover the pressure sensing lumen. The propensity for the pressure sensing lumen to become blocked by water is exacerbated by the narrow gauge of the lumen and its connected tubing, which causes liquid to enter the tubing by means of capillary action.
Several different solutions have been developed in an attempt to overcome this deficiency of differential-pressure flow sensors. One alternative has been to insert a heating electrode into the flow sensor so as to heat the inner surface of the sensor and thereby prevent water condensation. This technique requires the addition of electrical wiring and machinery to the flow sensor and ventilator, thus increasing the cost and mechanical complexity of the sensor. A second alternative has been to try ensure that the sensor remains oriented in space in such a way that the pressure-sensing lumen is on the superior aspect of the cylindrical body, rather than the dependent aspect where condensed water will accumulate due to gravity. This alternative has proven to be impractical, as movement of the patient or the flexible respiratory tubing inevitably results in movement of the flow sensor, and thus movement of the accumulated water within the sensor, causing blockage of the pressure-sensing lumen.
There is therefore a need for, and it would be highly advantageous to have, a differential-pressure flow sensor that prevents condensation of water vapor in the pressure-sensing lumens of the sensor, and that prevents blockage of the pressure-sensing lumens by condensed water which may flow into the sensor from the respiratory tubing. It would be desirable for such a sensor to achieve these aims without the addition of electrical components to the sensor.